Magnetic resonance (MR) imaging provides excellent soft tissue contrast and when used to guide focused ultrasound (FUS), providing the ability to localize, plan, monitor and verify treatments. FUS has been used to non-invasively treat uterine fibroids as well as breast, prostate, liver and brain cancer. As FUS can deliver large amounts of energy over a short time interval, the monitoring of treatments requires a high temporal resolution. Also, because the energy is delivered from a large transducer aperture to a small focus, a large field of view is required to monitor any possible energy deposition. The FUS beam will likely travel through several different tissue types during treatment where a portion of the beam will be reflected and transmitted at each tissue interface depending on the impedance difference between the tissues. Each tissue type will also absorb a different amount of the ultrasound energy. For example, 90% of the ultrasound energy through the skull is reflected or absorbed.
Monitoring of interventional treatments can be done using 2D or 3D MRI sequences where the method chosen is often governed by the trade-off between the needed temporal and spatial resolution and required field of view. Currently, clinical monitoring of MR guided FUS (MRgFUS) treatments is limited to a single (or relatively few) 2D slices providing a limited field of view. For example, 2D monitoring of the ultrasound focus during transcranial MRgFUS treatments cannot measure any heating near the skull surface, and will miss any unintended energy deposition due to beam aberration, or near field grating lobes.
MR temperature imaging does have some limitations, which are more apparent when using 2D imaging. Partial volume effects cause an underestimation of the actual temperature, which will increase with voxel size. It can also be difficult to properly position a single 2D slice to capture the entire focus. Multiple 2D slices will have a gap between each slice, meaning any temperature increase in the gap will not be measured. Respiration and motion artifacts will also introduce errors to the temperature monitoring.
3D MR thermometry can overcome many of the field of view, partial volume, and coverage gap limitations, which are inherent in 2D imaging but unfortunately, standard 3D sequences typically require too much time to acquire k-space to be clinically viable. Temporal resolution can be increased by methods involving undersampling such as temporally constrained reconstruction, model predictive filtering, Kalman filtering, parallel imaging or using a sequence designed for increased speed such as segmented echo-planar imaging (seg-EPI).
While a 3D seg-EPI offers several advantages, it has limitations. The chemical shift artifact, field inhomogeneity, and field variation due to motion artifacts are increased due to the low bandwidth in the phase encoding direction. The chemical shift typically requires imaging with fat saturation, while the respiration artifact can be corrected to a limited extent depending on the orientation of the 3D slab. Increasing the EPI factor, or number of lines collected per TR, will increase the temporal resolution while further escalating the chemical shift and respiration artifacts and decreasing the signal to noise ratio (SNR). Seg-EPI sequences also typically have image distortions along the phase encode direction.
Non-Cartesian 3D sequences, such as stack of stars and stack of spirals, have several advantages. Projection sampling performs well with high levels of undersampling. The center of k-space is sampled every TR providing robustness to motion, as well as the ability to correct respiration artifacts through self-navigation. Using a golden angle (GA) increment improves the ability for angular undersampling, as a GA increment guarantees an optimal projection angle distribution for an arbitrary number of projection angles, and the irrational nature of the GA also lends itself to compressed sensing. The GA is also an optimal radial projection order when using k-space weighted image contrast (KWIC) to increase the temporal resolution. Combining radial sampling with Cartesian slice encoding in stack of stars (SOS) sequences allows for 3D imaging with these advantages. The temporal resolution can be further increased by taking advantage of partial Fourier sampling in the slice direction.
While the implementation of non-Cartesian sampling trajectories has historically had some difficulties, these issues have been largely overcome. Off-resonance artifacts produce blurring instead of uni-direcitonal shift, but a more uniform field and increased readout bandwidth can help decrease the blur. Errors in the gradient timing can produce significant artifacts, but several correction methods have been successfully implemented. Finally, efficient algorithms and computer hardware can significantly reduce the computation time required to grid the non-Cartesian measurements onto a Cartesian grid.
Many regions of the body have significant amounts of adipose tissue near where interventional treatments are performed (e.g. breast, uterus), which can affect image quality. The strength of the chemical shift artifact and SNR are both related to the readout bandwidth. As the readout bandwidth is decreased, the SNR and chemical shift artifact will both increase. A simple method to maintain SNR while decreasing the chemical shift artifact is to increase the readout bandwidth and acquire multiple echoes. The individual echoes will have lower SNR; however, data from each echo can be combined to increase the overall SNR of both the magnitude and phase information. Acquiring multiple echoes has the added benefit of allowing calculation of T2* and the initial signal magnitude, M (0), as well as separate water/fat images. T2 has been shown to have a linear relationship with temperature in adipose tissue and has been used as a measure of temperature to monitor near field heating. The signal magnitude also has a temperature dependence through its dependence on temperature-dependent T1.